The first step in understanding a transmission system is to define the basic broadband transmission unit. For analog systems, this would be a channel group, which prior to transmission is typically multiplexed into supergroups and mastergroups. For digital transmission, the basic unit is the DS1 signal. The DS1 signal developed by a digital channel bank (e.g., the D-3 Channel Bank, as disclosed in U.S. Pat. No. 4,059,731, issued Nov. 22, 1977 to J. H. Green and J. E. Landry) and transmitted over a T-1 transmission line (1.544 megabits per second) is, at present, the workhorse of the Bell System digital transmission network.
The format of the DS1 digital signal consists of 24 eight-bit words and one framing bit for a total of 193 bits per frame. The 24 words typically represent 24 separate and distinct messages deposited in 24 separate and distinct channels. The words are PCM (pulse code modulation) encoded and the least significant bit (i.e., eighth bit) of a channel is periodically dedicated for signaling purposes.
Considering the signaling in more detail, eight bits are used for transmitting digital information (e.g., PCM encoded voice) for each channel in five of every six frames, and the eighth bit is borrowed for signaling purposes in every sixth frame. To identify these signaling digits, signaling framing information is inserted in the bit stream in the framing bit position of every other frame, i.e., the subframes. The subframe pattern that is used is 111000111000111 . . . The signaling frame is defined as the frame which follows a transition in the subframe pattern. The algorithm used by a receiving terminal to extract signaling information is to monitor the subframe pattern for a transition (a 1 to 0 or 0 to 1) and to gate the eighth bit of each channel to signaling circuitry during the signaling frame that follows each transition.
So much for the DS1 signal itself. This signal format has served the system well up till now. However, the need for additional data channel capacity is becoming more critical in D-type (digital channel bank) systems that use the standard DS1 format. Furthermore, with the possible attractive use of D-type systems for loop plant applications, an additional data channel is useful for the transmittal of channel activity (i.e., concentration). A proposal put forth heretofore has been to additionally time share (i.e., borrow) one or more bits (e.g., the eighth or least significant bit) of the information channels. This introduces unacceptable transmission impairments, however, and so has never been seriously pursued. Another proposal, first set forth in the article "D2 Channel Bank: Digital Functions" by A. J. Cirillo et al, Bell System Technical Journal, Volume 51, October 1972, pages 1701-1712, has been to substitute directly a data channel (CCIS) for the signaling subframe channel. This provides for a 4-kilobit per second data channel. The substitution of the eighth bit for signaling purposes would thus be discontinued and presumably the necessary signaling information carried in the CCIS data channel. This latter proposal, unfortunately, introduces a basic incompatibility into the digital network and it sacrifices signaling capabilities, e.g., overall signaling channel capacity is reduced.